1. Field of the Invention
This invention relates to a fixed length cell handling type image communication method as well as a transmission apparatus for fixed length cell handling type image communication and a reception apparatus for fixed length cell handling type image communication, and more particularly to a fixed length cell handling type image communication method as well as a transmission apparatus for fixed length cell handling type image communication and a reception apparatus for fixed length cell handling type image communication suitable for use with image communication which makes use of an ATM network which handles a fixed length cell called ATM (Asynchronous Transfer Mode) cell.
2. Description of the Related Art
FIG. 53 shows in block diagram an example of an image communication system which makes use of an ATM network. Referring to FIG. 53, the image communication system shown includes an ATM exchange 101, a plurality of (two in FIG. 53) ATM image communication apparatus 102, a plurality of (two in FIG. 53) cameras 104 for image communication, and a plurality of (two in FIG. 53) television sets 105.
The cameras 104 and the television sets 105 are normally connected to the respective ATM image communication apparatus 102 and accommodated in the ATM exchange 101 via respective user network interfaces (UNIs) including a plurality of channels. However, they may be accommodated directly in the ATM exchange 101 via an ATM image communication section 1012 in the ATM exchange 101, for example, like a camera 104xe2x80x2 and a television set 105xe2x80x2.
In the image communication system constructed in such a manner as described above, an image from each camera 104 (or 104xe2x80x2) is first subject to required processing such as image compression, conversion into an ATM cell (fixed length cell) and so forth by a corresponding one of the ATM image communication apparatus 102 (or ATM image communication section 1012) and then sent out as an ATM cell 106 to the ATM exchange 101. Then, an ATM switch section 1011 of the ATM exchange 101 is switched in response to a portion of a frame format of the ATM cell 106 which represents information of a destination of data so that the data are outputted to the desired transfer destination (in this instance, to one of the television sets 105 and 105xe2x80x2).
In the following, an image compression process (system), an ATM cell transferring process and so forth of the ATM image communication apparatus 102 or the ATM image communication section 1012 described above will be described in detail.
1-1. Outline
Generally, a compression method which utilizes a JPEG (Joint Photograph coding Experts Group) algorithm is well known as an image compression method. The JPEG algorithm has been produced as standards for image compression of the international standardization group JPEG.
FIG. 54 shows in block diagram an example of a construction of the ATM image communication apparatus 102 (or ATM image communication section 1012) which utilizes a JPEG system. Referring to FIG. 54, the ATM image communication apparatus 102 shown includes a transmission section 111 and a reception section 112. The transmission section 111 includes an 8xc3x978 blocking section 113, a discrete cosine transform section 114, a quantization section 115, a quantization table 116 and a JPEG coding section 117. The reception section 112 includes a JPEG decoding section 121, a dequantization section 122, a quantization table 123, an inverse discrete cosine transform section 124 and an 8xc3x978 deblocking section 125.
In the transmission section 111, the 8xc3x978 blocking section 113 performs 8xc3x978 blocking of image information for one field, that is, for image information for one screen. The discrete cosine transform section 114 performs discrete cosine transform (DCT) for extracting frequency components representative of characteristics of fineness of an image for each block of image information blocked by the 8xc3x978 blocking section 113. It is to be noted that image information generally includes higher frequencies at portions of finer patterns and includes lower frequencies at portions of rougher patterns.
The quantization section 115 performs quantization for each block, for which discrete cosine transform has been performed by the discrete cosine transform section 114, using the quantization table 116. Upon such quantization, the compression ratio of image data can be adjusted using a parameter called scaling factor SF. The JPEG coding section 117 performs JPEG coding processing by coding each block quantized by the quantization section 115 and outputs compressed data.
Meanwhile, in the reception section 112, the JPEG decoding section 121, dequantization section 122, quantization table 123, inverse discrete cosine transform section 124 and 8xc3x978 deblocking section 125 perform processing reverse to that performed by the JPEG coding section 117, quantization table 116, quantization section 115, discrete cosine transform section 114 and 8xc3x978 blocking section 113 described above, respectively. Consequently, the reception section 112 can regenerate original image data from compressed data transmitted thereto as JPEG coded compressed data as described above.
In the ATM image communication apparatus 102 (or ATM image communication section 1012) having the construction described above, input image data for one field, that is, original image information for one screen, is subject to blocking processing in units of vertical and horizontal 8xc3x978 pixels (8xc3x978 blocking) by the 8xc3x978 blocking section 113. Further, each block obtained by such blocking is subject to discrete cosine transform by the discrete cosine transform section 114 so that frequency components included in the block are extracted.
Thereafter, each block discrete cosine transformed in this manner is quantized by the quantization section 115 using the quantization table 116 so that the image is compressed. The scaling factor SF is adjusted to adjust the compression ratio then. It is to be noted that, where an input image is the same, generally a decrease of the scaling factor SF decreases the compression ratio and increases the amount of image data after compression, but an increase of the scaling factor SF increases the compression ratio and decreases the amount of image data after compression.
Further, when image compression is performed, generally the amount of data after compression is large with an image of a fine pattern, but is small with an image of a rough pattern. Further, where an image is the same, if the compression ratio is raised to decrease the amount of data after compression by means of the scaling factor SF, then the picture quality after decompression (regeneration) is deteriorated, but if the compression ratio is lowered to increase the amount of data after compression, then the picture quality after regeneration is improved.
Then, each block quantized in such a manner as described above is coded by the JPEG coding section 117 and outputted as JPEG compressed image data.
On the other hand, JPEG image data received by the reception section 112 are subject to processing reverse to that of the transmission section 111 by the JPEG decoding section 121, dequantization section 122, quantization table 123, inverse discrete cosine transform section 124 and 8xc3x978 deblocking section 125 so that original image data are regenerated.
It is to be noted that FIG. 55 shows an example of a frame format employed in the JPEG system mentioned above. Referring to FIG. 55, the JPEG frame format shown includes an SOI (Start Of Image marker), a quantization table 118, a compressed image data part 119 and an EOI (End Of Image marker). The SOI is a code indicating the beginning of an image data frame, and a fixed value of, for example, xe2x80x9cFFD8xe2x80x9d is placed in the code. Meanwhile, the EOI is a code indicating the end of the image data frame, and another fixed value of, for example, xe2x80x9cFFD9xe2x80x9d is placed in the code.
The quantization table 118 represents data including a scaling factor SF described above which defines a compression ratio and is used as the quantization table 116 or 123 shown in FIG. 54 upon quantization by the quantization section 115 during image compression or upon dequantization by the dequantization section 122 during image decompression. The compressed image data part 119 can include image data for one screen after compression.
1-2. 8xc3x978 Blocking/Deblocking
Subsequently, 8xc3x978 blocking/deblocking by the 8xc3x978 blocking section 113 and the inverse discrete cosine transform section 124 described above will be described in detail.
First, in order to allow quantization processing by the quantization section (DCT-Based Encoder) 115 when an image is inputted, input image data for one field are subject to blocking (8xc3x978 blocking) processing in units of vertical and horizontal 8xc3x978 pixels to form blocks (Block) by the 8xc3x978 blocking section 113, for example, as seen in FIG. 56. On the other hand, upon decoding of an image, since the output of the dequantization section (DCT-Based Decoder) 122 is in units of a block, 8xc3x978 deblocking is performed by the 8xc3x978 deblocking section 125 to regenerate original (source) image data for one field.
It is to be noted that, in this instance, 8xc3x978 pixels of each block (Blocki) can individually be represented by Syx as seen, for example, in FIG. 57.
1-3. Discrete Cosine Transform
The discrete cosine transform section 114 performs, for original image data (Source Image Samples) Syx blocked in 8xc3x978 pixels, two-dimensional discrete cosine transform (FDCT: Forward Discrete Cosine Transform) given by the equation (1) given below so that DCT coefficients SVU representative of frequency components included in each block are determined in units of one pixel as seen, for example, in FIG. 58.                               S          VU                =                                            (                                                2                                                  N                                            )                        2                    ⁢                      C            U                    ⁢                      xe2x80x83                    ⁢                      C            V                    ⁢                                    ∑                              X                =                0                                            N                -                1                                      ⁢                                          ∑                                  X                  =                  0                                                  N                  -                  1                                            ⁢                                                S                  yx                                ⁢                                  cos                  ⁡                                      (                                                                                            (                                                                                    2                              ⁢                              x                                                        +                            1                                                    )                                                ⁢                        U                        ⁢                                                  xe2x80x83                                                ⁢                        π                                                                    2                        ⁢                        N                                                              )                                                  ⁢                                  cos                  ⁡                                      (                                                                                            (                                                                                    2                              ⁢                              x                                                        +                            1                                                    )                                                ⁢                        V                        ⁢                                                  xe2x80x83                                                ⁢                        π                                                                    2                        ⁢                        N                                                              )                                                                                                          (        1        )            
where Uxe2x80x2, V=0, 1, . . . , Nxe2x88x921, and CU, CV=1{square root over (2)} (when U, V=0) or CU, CV=1 (when U, Vxe2x89xa00). In this instance, since image data are processed in units of 8xc3x978 blocks, N=8. Accordingly, the equation (1) above is re-written as the following equation (2):                               S          VU                =                              (                          1              4                        )                    ⁢                      C            U                    ⁢                      xe2x80x83                    ⁢                      C            V                    ⁢                                    ∑                              X                =                0                            7                        ⁢                                          ∑                                  X                  =                  0                                7                            ⁢                                                S                  yx                                ⁢                                  cos                  ⁡                                      (                                                                                            (                                                                                    2                              ⁢                              x                                                        +                            1                                                    )                                                ⁢                        U                        ⁢                                                  xe2x80x83                                                ⁢                        π                                            16                                        )                                                  ⁢                                  cos                  ⁡                                      (                                                                                            (                                                                                    2                              ⁢                              x                                                        +                            1                                                    )                                                ⁢                        V                        ⁢                                                  xe2x80x83                                                ⁢                        π                                            16                                        )                                                                                                          (        2        )            
On the other hand, the inverse discrete cosine transform section 124 performs inverse DCT transform represented by the equation (3) given below for the output of the dequantization section 122 to determine quantized DCT coefficients SqVU as seen, for example, in FIG. 59.                               S          VU                =                                            (                                                2                                                  N                                            )                        2                    ⁢                                    ∑                              X                =                0                                            N                -                1                                      ⁢                                          ∑                                  X                  =                  0                                                  N                  -                  1                                            ⁢                                                C                  U                                ⁢                                  xe2x80x83                                ⁢                                  C                  V                                ⁢                                  xe2x80x83                                ⁢                                  R                  VU                                ⁢                                  xe2x80x83                                ⁢                                  cos                  ⁡                                      (                                                                                            (                                                                                    2                              ⁢                              x                                                        +                            1                                                    )                                                ⁢                        U                        ⁢                                                  xe2x80x83                                                ⁢                        π                                                                    2                        ⁢                        N                                                              )                                                  ⁢                                  cos                  ⁡                                      (                                                                                            (                                                                                    2                              ⁢                              x                                                        +                            1                                                    )                                                ⁢                        V                        ⁢                                                  xe2x80x83                                                ⁢                        π                                                                    2                        ⁢                        N                                                              )                                                                                                          (        3        )            
Also in this instance, since image data are processed in units of 8xc3x978 blocks, N=8, and accordingly, the equation (3) above is re-written as the following equation (4):                               S          VU                =                              (                          1              4                        )                    ⁢                                    ∑                              X                =                0                            7                        ⁢                                          ∑                                  X                  =                  0                                7                            ⁢                                                C                  U                                ⁢                                  xe2x80x83                                ⁢                                  C                  V                                ⁢                                  xe2x80x83                                ⁢                                  R                  VU                                ⁢                                  xe2x80x83                                ⁢                                  cos                  ⁡                                      (                                                                                            (                                                                                    2                              ⁢                              x                                                        +                            1                                                    )                                                ⁢                        U                        ⁢                                                  xe2x80x83                                                ⁢                        π                                            16                                        )                                                  ⁢                                  cos                  ⁡                                      (                                                                                            (                                                                                    2                              ⁢                              x                                                        +                            1                                                    )                                                ⁢                        V                        ⁢                                                  xe2x80x83                                                ⁢                        π                                            16                                        )                                                                                                          (        4        )            
1-4. Quantization
The quantization table 116 (118) used by the quantization section 115 described above is provided for quantization of 64 DCT coefficients SVU which are a result of calculation of FDCT transform for each block by the JPEG coding section 117, and includes quantization step sizes QVU (Quantization Table) for 64 DCT coefficients SVU set therein as seen, for example, in FIG. 58.
The quantization section 115 thus performs calculation defined by the equation (5) given below based on the DCT coefficients SVU and the quantization step sizes QVU corresponding to the DCT coefficients SVU to determine quantized DCT coefficients SqVU representative of a compressed image data amount as seen in FIG. 58.
SqVU=round(SVU/QVU)xe2x80x83xe2x80x83(5)
where round( ) represents a function which assumes a value nearest to the value of ( ) (half-adjust).
1-5. Dequantization
On the other hand, the quantization table 123 (118) in the dequantization section 122 is the same as the quantization table 116 (118) of the quantization section 115 on the other party side of image communication. In this instance, based on quantized DCT coefficients (Received Quantized DCT Coefficients) SqVU received via the JPEG decoding section 121 and the quantization step sizes QVU corresponding to the quantized DCT coefficients SqVU, calculation defined by the equation (6) given below is performed to obtain dequantized DCT coefficients RVU.
RVU=SqVUxc3x97QVUxe2x80x83xe2x80x83(6)
1-6. Quantization Table
By the way, the quantization step sizes QVU of the quantization table 118 described above are calculated based on values qVU of, for example, such a standard quantization table 118xe2x80x2 as shown in FIG. 60 and a scaling factor SF by calculation defined by the following equation (7):
QVU=2557∇[round(qVUxe2x88x92SF)]xe2x80x83xe2x80x83(7)
where round( ) represents conversion into an integer by half-adjust similarly as in the equation (5), and 2557∇ [ ] represents a function of rounding an integer higher than 255 to 255. It is to be noted that, since the standard quantization table 118xe2x80x2 shown in FIG. 60 presents higher values qVU in the directions of V and U, upon quantization, comparatively high frequencies included in each block (image portions having comparatively fine patterns) are liable to be rounded.
Then, the values QVU of the quantization table 118 calculated using the equation (7) given above are outputted as data for the quantization table 118 of the JPEG frame format described hereinabove with reference to FIG. 55.
In order to obtain a good picture quality, the scaling factor SF mentioned above should normally be set to a value approximately equal to 1, and where the scaling factor SF is lower than 1, the picture quality is improved, but image data after compression exhibit an increased amount. On the contrary, where the scaling factor SF is higher than 1, the picture quality is deteriorated, but image data after compression exhibit a decreased amount. Accordingly, in order to keep a good picture quality, it is necessary to use a scaling factor SF having a value as low as possible within an allowable range of the amount of image data.
In the following, an ATM cell in which image data after compression are included will be described in detail.
2-1. Format of ATM Cell
FIG. 61 shows an example of the format of an ATM cell. Referring to FIG. 61, the ATM cell includes an ATM header (logic channel information part) 131 of 5 bytes in which an address (connection address) of a transfer destination of data is included, and a data part 132 of 48 bytes called SAR-PDU (Segmentation and Reassembly-Protocol Data Unit). The last one byte of the ATM header 131 includes an HEC (Header Error Control) for detecting a bit error of the header.
By the way, a service by provision of the AAL (ATM Adaptation Layer) type 1 is used to provide a constant information rate (CBR: Constant Bit Rate) service of voice, an image or the like by an ATM. In the AAL type 1, the SAR-PDU mentioned above has, for example, such a format as seen in FIG. 62. Referring to FIG. 62, the first 1 byte of the SAR-PDU is a SAR header, and the remaining 47 bytes are a SAR-SDU (Segmentation And Reassembly-Service Data Unit).
The SAR header is used to indicate a sending out order of a cell and is used, on the reception side, for detection of a missing cell. Meanwhile, the SAR-SDU is used as an information field and includes information of the CBR service of sound, an image or the like to be sent out. It is to be noted that the data are successively sent out in the order from the left to the right and from above to below in FIG. 62.
FIG. 63 shows an example of the format of the SAR header mentioned above. Referring to FIG. 63, the SAR header includes an SNF (Sequence Number Field) of 4 bits and an SNPF (Sequence Number Protect Field) of 4 bits. Further, the SNF includes a CS bit indicating a CS (Conversience Sub-layer) and a SN (Sequence Number) of 3 bits which represents a number from 0 to 7 in a binary number and is cyclically incremented to indicate a sending out order of the cell. The SNPF includes a CRC (Cyclic Redundancy Check) indicating a CRC calculation value for error detection and correction of the SN, and an EP (Even Parity) bit representing an even parity.
Then, in the CBR service, data of sound, an image or the like to be transmitted at a fixed rate are divided for each 47 bytes to produce such an SAR-SDU as described above, and an SAR header for indication of a sending out order of the cell is added to the SAR-SDU. Further, an ATM header indicating a destination address is added to the SAR-SDU, and a resulting SAR-SDU is sent out to a network.
On the reception side, the SAR-SDU (data part) of 47 bytes is extracted from the received ATM cell to regenerate data of sound, an image or the like at a fixed rate.
2-2. SRTS (Synchronous Residual Time Stamp) Method
The SRTS method is a kind of a Source Clock Frequency Recovery Method and uses an RTS (Residual Time Stamp) obtained by sampling a reference clock (Common Reference Clock) signal obtained by dividing a network clock (Network Clock) signal extracted from reception data received from a UNI using a service clock (Service Clock) signal extracted from reception data received from a service interface. It is to be noted that the same clock signal from the UNI is used commonly by the transmission section 111 and the reception section 112.
FIG. 64 illustrates an example of RTS data production and transfer processing. Referring to FIG. 64, for example, a network clock frequency fn is divided to xc2xdX (X is an integer) by a binary counter 141 so that a reference clock frequency fnx is produced. Here, the integer X is set so that the relationship thereof to a transmission user clock frequency fs may satisfy 1xe2x89xa6fnx/fs less than 2.
Further, a counter (Ct) 142 is a P-bit counter (in this instance, P=4) and operates in synchronism with successive clocks extracted from a network clock signal by the binary counter 141. An output of the counter 142 is sampled for each N (N=3,008) service clock cycles from a binary counter 143 by a DFF 144. As a result, a 4-bit output of the DFF 144 is produced as the RTS described above.
Then, the 4-bit RTS is transferred by the CSI bits in successive SAR-PDU headers. It is to be noted that a frame construction for each 8 bits is provided by an SC value of the modulo (mod) 8. Here, the four bits of SC=1, 3, 5 and 7 from among 8 bits of the SCI bits described above are allocated to the RTS while the remaining 4 bits are xe2x80x9c0xe2x80x9d. In other words, the SAR-PDU headers of the odd-numbered SC values=1, 3, 5 and 7 are used for RTS transfer. The MSB (most significant bit) of the RTS (that is, RTS4) is positioned at the CSI bit of the SAR-PDU header of SC=1, and the remaining bits of the RTS are positioned similarly in order as seen in FIG. 64.
Subsequently, an NTSC signal will be described in detail.
The NTSC signal is an image signal of international standards determined by the National Television System Committee and is generally used also as input/output signals of a television set or a video apparatus for public welfare.
One frame which is information for one screen usually includes 525 lines, and screen information of approximately 30 frames is transmitted for one second. In this instance, however, since a frame of information of each screen is divided into and transmitted in an even-numbered field and an odd-numbered field as seen, for example, in FIG. 65, the number of fields transmitted for one second is approximately 60. By successively transmitting still pictures of approximately 60 fields for one second as an NTSC signal, transmission and display of moving pictures are performed.
FIG. 66(a) illustrates an example of an NTSC signal. Referring to FIG. 66(a), the NTSC signal shown is an analog signal in which an image signal (field image signal), such a vertical synchronizing signal (VSYN) as shown in FIG. 66(b) and such a horizontal synchronizing signal (HSYN) as shown in FIG. 66(c) are frequency multiplexed.
It is to be noted that, for the natural frequency of image data, the following frequency is usually used.
The basic sampling clock frequency fe in xe2x80x9cEncoding parameters of digital television for studios: CCIR REC. 601.2 (1993.7)xe2x80x9d is
fe=13.5 (MHz)
Further, the frequency fH of the horizontal synchronizing signal, the frequency fV of the vertical synchronizing signal and the basic sampling clock fe have the following relationship:
fH=fe/858
fV=2fH/525
In the following, various image compression methods which may be used by the ATM image communication apparatus 102 (or ATM image communication section 1012) will be described in detail. It is to be noted that, for the convenience of description, description will be given also of a non-compression method by which image data are not compressed.
{circle around (1)} Non-Compression Method
FIG. 67 shows in block diagram of the ATM image communication apparatus 102 (or ATM image communication section 1012) where a non-compression method is employed. Referring to FIG. 67, the ATM image communication apparatus 102 shown includes, as a transmission section 111A thereof, an NTSC reception section 151A for receiving an NTSC signal from a camera 104 (or 104xe2x80x2), an analog to digital (A/D) conversion section 152A, an image data production section 153A and an ATM transmission section 154A. The ATM image communication apparatus 102 further includes, as a reception section 112A thereof, an ATM reception section 155A, an image data separation section 156A, a digital to analog (D/A) conversion section 157A and an NTSC transmission section 158A for transmitting an NTSC signal for a television set 105 (105xe2x80x2).
In the ATM image communication apparatus 102 (or ATM image communication section 1012) having the construction described above, an NTSC signal from the camera 104 (or 104xe2x80x2) is received by the NTSC reception section 151A, by which timing signals such as a horizontal synchronizing signal are extracted. Then, the NTSC signal is converted into a digital signal by the analog to digital conversion section 152A, and digital image data are produced from an output of the analog to digital conversion section 152A by the image data production section 153A.
Since no image compression has been performed for the image data, the amount of data for one field does not vary among different fields. Accordingly, the ATM transmission section 154A can transmit the image data by ATM cells of the AAL type 1. In particular, transmission image data are divided into SAR-SDUs in units of 47 bytes, and an SAR header and an ATM header are added to each SAR-SDU to produce an ATM cell. The thus produced ATM cells are transmitted to an ATM-UNI. In the reception section 112A, processing reverse to that performed by the transmission section 111A is performed by the ATM reception section 155A, image data separation section 156A, digital to analog conversion section 157A and NTSC transmission section 158A.
In the non-compression method described above, since moving picture data are converted directly into ATM cells without compressing the same in this manner, the circuit construction is simple comparing with other methods which will be described below, and besides a good picture quality can be realized. Further, since compression is not involved, the time for image compression processing is not required. Accordingly, the end-to-end transmission delay time including image compression is as short as approximately several ms, and the non-compression method is suitable for real-time communication.
However, since the non-compression method involves no compression of image data, the transmission rate of image data must be approximately 100 Mbit/s to 200 Mbit/s and cannot make effective use of an ATM-UNI line, and besides is much disadvantageous in terms of the communication cost. Therefore, the following various compression methods have been proposed.
{circle around (2)} Differential Compression (Decompression) Method FIG. 68 shows in block diagram a construction of the ATM image communication apparatus 102 (or ATM image communication section 1012) where a differential compression method is employed. Referring to FIG. 68, the ATM image communication apparatus 102 shown includes, as a transmission section 111B thereof, an NTSC reception section 151B for receiving an NTSC signal from a camera 104 (or 104xe2x80x2), an analog to digital (A/D) conversion section 152B, a differential compression section 153B, an image data production section 154B and an ATM transmission section 155B. The ATM image communication apparatus 102 further includes, as a reception section 112B thereof, an ATM reception section 156B, an image data separation section 157B, a differential decompression section 158B, a digital to analog (D/A) conversion section 159B and an NTSC transmission section 160B for receiving an NTSC signal for a television set 105 (or 105xe2x80x2).
In the ATM image communication apparatus 102 (or ATM image communication section 1012) having the construction described above, an NTSC signal (image signal) from the camera 104 (or 104xe2x80x2) is supplied to the NTSC reception section 151B, by which timing signals such as a horizontal synchronizing signal are extracted from the NTSC signal. Then, the NTSC signal is converted into a digital signal by the analog to digital conversion section 152B and then converted into and compressed as a differential signal by the differential compression section 153B.
Here, since the differential signal is lower in level than the original image signal as seen, for example, from FIG. 69, the bit amount thereof is reduced to approximately one half that of the original image signal. Further, the NTSC signal differentially compressed in this manner is inputted to the image data production section 154B, by which digital image data are produced based on the differential compressed signal.
Then, the image data are converted into ATM cells of the AAL type 1 by the ATM transmission section 155B and transmitted as such ATM cells. In particular, the transmission image data are divided into SAR-SDUs in units of 47 bytes, and an SAR header and an ATM header are added to each SAR-SDU to produce an ATM cell. The thus produced ATM cells are transmitted to an ATM-UNI. In the reception section 112B, processing reverse to that performed by the transmission section 111B is performed by the ATM reception section 156B, image data separation section 157B, differential decompression section 158B, digital to analog (D/A) conversion section 159B and NTSC transmission section 160B so that the original images (moving pictures) are reproduced.
In the ATM image communication apparatus 102 (or ATM image communication section 1012) where the differential compression method described above is employed, since differential information of YC signals of moving pictures for one field is converted into ATM cells, although the circuit construction is complicated rather than another apparatus which employs the non-compression method, a good picture quality can be obtained by regeneration of data by a construction simpler than those of the other methods which will be described below. Further, with the differential compression method, since the image compression processing time is comparatively short, the end-to-end transmission delay time including image compression can be made as short as approximately several ms. Accordingly, also the present method is suitable for real time communication.
However, also the present method is disadvantageous in terms of effective utilization of an ATM-UNIT line and the communication cost since the transmission rate of image data is still as high as approximately 50 Mbit/s. Further, since the differential compression does not allow editing for each field, it is not suitably used for a source for broadcasting.
{circle around (3)} H261/MPEG1 System
Subsequently, the H261/MPEG (Motion Picture image coding Experts Group) 1 system will be described. The H261/MPEG 1 (compression) system has a close relationship to the JPEG for transmission of still pictures. In the H261/MPEG 1 system, transform basically using DCT is performed for signals for one field and then processing based on a correlation between fields is performed to further improve the compression ratio.
FIG. 70 shows in block diagram a construction of the ATM image communication apparatus 102 (or ATM image communication section 1012) where an H261/MPEG 1 system is employed. Referring to FIG. 70, the ATM image communication apparatus 102 shown includes, as a transmission section 111C thereof, an NTSC reception section 151C for receiving an NTSC signal from a camera 104 (or 104xe2x80x2), an analog to digital (A/D) conversion section 152C, an H261/MPEG 1 compression section 153C, an image data production section 154C and an ATM transmission section 155C. Further, the ATM image communication apparatus 102 includes, as a reception section 112C thereof, an ATM reception section 156C, an image data separation section 157C, an H261/MPEG 1 decompression section 158C, a digital to analog (D/A) conversion section 159C and an NTSC transmission section 160C for transmitting an NTSC signal for a television set 105 (or 105xe2x80x2).
In the ATM image communication apparatus 102 (or ATM image communication section 1012) having the construction described above, an NTSC signal from the camera 104 (or 104xe2x80x2) is received by the NTSC reception section 151C, by which timing signals such as a horizontal synchronizing signal are extracted from the NTSC signal. Then, the NTSC signal is converted into a digital signal by the analog to digital conversion section 152C and then inputted to the H261/MPEG 1 compression section 153C.
The H261/MPEG 1 compression section 153C performs compression of the digital NTSC signal basically using DCT transform, calculates a correlation between fields and, in order to make the data amount fixed, feeds back the correlation to effect H261/MPEG 1 compression.
The image data production section 154C produces H261/MPEG 1 frames based on the H261/MPEG 1 compressed data. Then, since the H261/MPEG 1 frames have fixed data amounts in average, the ATM transmission section 155C can transmit the image data by ATM cells of the AAL type 1. In particular, the transmission image data are divided into SAR-SDUs in units of 47 bytes, and an SAR header and an ATM header are added to each SAR-SDU to produce an ATM cell. The thus produced ATM cells are transmitted to an ATM-UNI. In the reception section 112C, processing reverse to that performed by the transmission section 111C described above is performed to regenerate the original images.
In this manner, the H261/MPEG 1 system described above exhibits such a high compression ratio that images can be transmitted at the transmission rate of approximately several Mbit/sec.
With the H261/MPEG 1 system, however, the circuit construction is complicated, and since a correlation between fields is calculated, the image compression processing time is as long as approximately several hundreds ms and also the processing time on the regeneration (decompression) side is long. Consequently, the end-to-end transmission delay time including image compression is approximately 400 ms. Therefore, although the H261/MPEG 1 system is suitably used for storage of images or with a cable television, it is not suitable for real time communication such as for communication for a television conference.
Further, since the target of the picture quality is approximately 320xc3x97240 pixels, the picture quality is lower than xc2xd that of a television broadcast, and a sufficient picture quality cannot be obtained.
{circle around (4)} MPEG 2 System
Subsequently, the MPEG 2 system will be described. The MPEG 2 (compression) system has been developed so that the picture quality of compressed image data after regeneration may be equal to or higher than that of a television broadcast. The MPEG 2 system itself has a close relation to the JPEG or MPEG 1 system described above. In the MPEG 2 system, signals for one field are transformed basically using DCT transform and then processing based on a correlation between fields is performed in order to further improve the compression ratio.
FIG. 71 shows in block diagram a construction of the ATM image communication apparatus 102 (or ATM image communication section 1012) where an MPEG 2 system is employed. Referring to FIG. 71, the ATM image communication apparatus 102 shown includes, as a transmission section 111D thereof, an NTSC reception section 151D for receiving an NTSC signal from a camera 104 (or 104xe2x80x2), an analog to digital (A/D) conversion section 152D, an MPEG 2 compression section 153D, an image data production section 154D and an ATM transmission section 155D. The ATM image communication apparatus 102 further includes, as a reception section 112D thereof, an ATM reception section 156D, an image data separation section 157D, an MPEG 2 decompression section 158D, a digital to analog (D/A) conversion section 159D, and an NTSC transmission section 160D for transmitting an NTSC signal for a television set 105 (or 105xe2x80x2).
In the ATM image communication apparatus 102 (or ATM image communication section 1012) having the construction described above, an NTSC signal from the camera 104 (or 104xe2x80x2) is received by the NTSC reception section 151D, by which timing signals such as a horizontal synchronizing signal are extracted. Then, the NTSC signal is converted into a digital signal by the analog to digital conversion section 152D and then inputted to the MPEG 2 compression section 153D.
The MPEG 2 compression section 153D performs transform of the digital NTSC signal basically using DCT transform, calculates a correlation between fields, and, in order to make the data amount fixed, feeds back the correlation to perform MPEG 2 compression. Then, based on the MPEG 2 compressed data, MPEG 2 frames are produced by the image data production section 154D.
Since the MPEG 2 frames have fixed data amounts in average, the ATM transmission section 155D can transmit the image data by ATM cells of the AAL type 1. In particular, the transmission image data are divided into SAR-SDUs in units of 47 bytes, and an SAR header and an ATM header are added to each SAR-SDU to produce an ATM cell. The thus produced ATM cells are transmitted to an ATM-UNI. Also in this instance, in the reception section 112D, processing reverse to that performed by the transmission section 111D is performed to regenerate the original pictures.
In this manner, the MPEG 2 system described above exhibits a high compression ratio and can transmit image data at a transmission rate of approximately 10 Mbit/sec, and besides the picture quality after regeneration is high.
With the MPEG 2 system, however, since the compression and decompression circuits used are very complicated in circuit construction and expensive and particularly the compression section 153D is formed as a very large apparatus, the overall cost becomes very high. Further, also with the MPEG 2 system, since a correlation between fields is calculated, the image compression processing time is as long as approximately several hundreds ms and also the processing time on the decompression side is long. Consequently, the end-to-end transmission delay time including image compression is approximately 400 ms, and accordingly, the MPEG 2 system is not suitable for real time communication such as for communication for a television conference.
{circle around (5)} JPEG System
Subsequently, the JPEG system mentioned hereinabove will be described in more detail. While the JPEG (compression) system has a close relation to the MPEG compression system described above, it originally is a method for use for transmission of still pictures. In the JPEG system, JPEG compression processing is performed for signals still of pictures for one field basically using DCT, and then the signals are converted into and transmitted as ATM cells.
FIG. 72 shows in block diagram a construction of the ATM image communication apparatus 102 (or ATM image communication section 1012) where a JPEG system is employed. Referring to FIG. 72, the ATM image communication apparatus 102 shown includes, as a transmission section 111E thereof, an NTSC reception section 151E for receiving an NTSC signal from a camera 104 (or 104xe2x80x2), an analog to digital (A/D) conversion section 152E, a JPEG compression section 153E, an image data production section 154E and an ATM transmission section 156E. The ATM image communication apparatus 102 further includes, as a reception section 112E thereof, an ATM reception section 156E, an image data separation section 157E, a JPEG decompression section 158E, a digital to analog (D/A) conversion section 159E, and an NTSC transmission section 160E for transmitting an NTSC signal for a television set 105 (or 105xe2x80x2).
In the ATM image communication apparatus 102 (or ATM image communication section 1012) having the construction described above, an NTSC signal from the camera 104 (or 104xe2x80x2) is received by the NTSC reception section 151E, by which timing signals such as a horizontal synchronizing signal are extracted. Then, the NTSC signal is converted into a digital signal by the analog to digital conversion section 152E and then inputted to the JPEG compression section 153E.
The digital NTSC signal is JPEG compressed by the JPEG compression section 153E and then inputted to the image data production section 154E, by which JPEG frames are produced. Then, the JPEG frames are converted into and transmitted as ATM cells by the ATM transmission section 155E.
It is to be noted that in the JPEG compression system, the compression ratio of images is adjusted with a parameter called scaling factor as described hereinabove. Further, even if the scaling factor is equal, since the data amount of image data per one field depends upon the picture quality, the ATM transmission section 155E cannot usually transmit image data by ATM cells of the AAL type 1.
Accordingly, the JPEG compression system is principally used for transmission of still pictures or for transmission of storage data which need not be transmitted at a fixed rate. Further, although the compression ratio in the JPEG compression system, is not as high as that of the MPEG 2 system, it is comparatively high, and if it can be used for transmission of moving pictures, then a picture quality equal to that obtained by the MPEG 2 system can be obtained at the transmission rate of approximately 20 Mbit/second.
Further, with the JPEG system, since also the circuit construction is simpler than that of the MPEG 2 compression system, a comparatively small apparatus can be reduced at a lower cost. Further, since the JPEG system does not involve calculation of a correlation between fields different from the MPEG 2, the image compression processing time is as very low as approximately several tens ms.
However, with the JPEG system, since it is directed to still pictures, the data amount of image data after compression processing is different for each one field depending upon the picture quality even if the scaling factor is equal (that is, even if the compression ratio is not varied). Consequently, the ATM transmission section 155E cannot usually transmit image data by ATM cells of the AAL 1 type. Accordingly, although the JPEG system can be used for transmission of still pictures or for transmission of storage data which need not be transmitted at a fixed rate, it is almost impossible to use the JPEG system for transmission of moving picture data.
It is an object of the present invention to provide a fixed length cell handling image communication method wherein moving picture data are compressed to data having a fixed transmission rate using a compression method for still pictures to obtain fixed length cells so that image processing such as compression/decompression of the moving picture data can be performed at a high rate while keeping a high compression ratio and a high picture quality.
It is another object of the present invention to provide a transmission apparatus for fixed length cell handling type image communication and a reception apparatus for fixed length cell handling type image communication which can perform image processing for moving picture data while keeping a high compression ratio and a high picture quality and which can be produced in a small size and at a reduced cost.
In order to attain the objects described above, according to an aspect of the present invention, there is provided a fixed length cell handling type image communication method wherein image data are communicated in fixed length cells each including a logical channel information part and a data part, comprising the steps of performing still picture compression processing for moving picture data, which define information of a plurality of screens individually divided into a plurality of fields to be successively transmitted per unit time, in response to field timing information representative of a compression timing of each field to convert the moving picture data into variable length data independent of each other, and converting the variable length data into fixed length data having a fixed transmission rate and sending out the fixed length data and the field timing information as the fixed length cells to transmit the moving picture data.
With the fixed length cell handling type image communication method, moving picture data can be compressed effectively using a compression method for still pictures and the moving picture data after such compression can be converted into fixed length cells readily and transmitted in an asynchronous fashion. Accordingly, a transmission line for fixed length cells can be utilized very efficiently. Further, in this instance, since the fixed length data and the field timing information are sent out in the fixed length cells, the moving picture data compressed using the compression method for still pictures can be regenerated with certainty and at a high rate on the reception side, and consequently, real time communication can be realized.
According to another aspect of the present invention, there is provided a transmission apparatus for fixed length cell handling type image communication wherein image data are transmitted as fixed length cells each including a logical channel information part and a data part, comprising an image data compression section for performing still picture compression processing for moving picture data, which define information of a plurality of screens individually divided into a plurality of fields to be successively transmitted per unit time, in response to field timing information representative of a compression timing of each field to convert the moving picture data into variable length data independent of each other, a transmission buffer section for converting the variable length data obtained by the image data compression section into fixed length data having a fixed transmission rate, and a fixed length cell transmission section for placing the fixed length data and the field timing information into the data parts of the fixed length cells and transmitting the fixed length cells.
With the transmission apparatus for fixed length cell handling type image communication, an apparatus to which the fixed length cell handling type image communication method described above is applied and in which the individual functions are performed divisionally by the respective components can be constructed very readily.
According to a further aspect of the present invention, there is provided a fixed length cell handling type image communication method wherein image data are communicated in fixed length cells each including a logical channel information part and a data part, comprising the steps of receiving fixed length cells including image-compressed fixed length data of a fixed transmission rate representing moving picture data which define information of a plurality of screens individually divided into a plurality of fields and field timing information representative of a compression timing of each field, regenerating the fixed length data and the field timing information and converting, based on the regenerated field timing information, the fixed length data into variable length data which are independent of each other for the individual fields, and performing still picture decompression processing for the variable length data to regenerate the original moving picture data.
With the fixed length cell handling type image communication method described above, moving picture data after compression in the form of fixed length cells can be received in an asynchronous fashion, and the moving picture data can be decompressed (regenerated) effectively using the decompression method for still pictures. Accordingly, a transmission line for the fixed length cells can be utilized very efficiently. Further, in this instance, since the fixed length data and the field timing information are received in the fixed length cells, the moving picture data compressed using the still picture compression method can be regenerated with certainty and at a high rate, and consequently, real time communication can be achieved.
According to a still further aspect of the present invention, there is provided a reception apparatus for fixed length cell handling type image communication wherein image data are transmitted as fixed length cells each including a logical channel information part and a data part, comprising a fixed length cell reception section for receiving fixed length cells including image-compressed fixed length data of a fixed transmission rate representing moving picture data which define information of a plurality of screens individually divided into a plurality of fields and field timing information representative of a compression timing of each field and regenerating the fixed length data and the field timing information from the fixed length cells, a reception buffer section for converting, based on the field timing information regenerated by the fixed length cell reception section, the fixed length data into variable length data which are independent of each other for the individual fields, and an image data decompression section for performing still picture decompression processing for the variable length data from the reception buffer section to regenerate the original moving picture data.
With the reception apparatus for fixed length cell handling type image communication, an apparatus to which the fixed length cell handling type image communication method described above is applied and in which the individual functions are performed divisionally by the respective components can be constructed very readily.
Further objects, features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference characters.